This invention relates to a pneumatic servo booster adapted for use in a hydraulic braking system of a vehicle.
Typical prior art pneumatic servo booster comprises a housing, a power piston unit partitioning the interior of the housing into front and rear chambers and having therein first and second passages, first ends of which opening respectively to front and rear chambers and second ends of which opening respectively to the interior of the power piston unit, a valve mechanism provided in the interior of the power piston unit and being associated with an input rod to control the communication between the first and second passages and the communication between the second passage and a source of a first reference pressure. The front chamber is connected to a source of a second reference pressure which is lower than the first reference pressure. The second reference pressure is a vacuum pressure, in one example, which is generated in an intake manifold of an engine and the first reference pressure may be the atmospheric pressure.
In actuating the input rod the communication between the first and second passages is cut off and the second passage is communicated with the source of the first reference pressure, whereby the first reference pressure is introduced into the rear chamber to produce a pressure difference between the rear and front chambers and the differential pressure acting on the power piston unit is taken out through an output rod. In releasing the input force applied on the input rod, the communication between the first reference pressure source and the second passage is cut off and, the second passage is communicated with the first passage whereby the relatively high pressure air in the rear chamber flows into the front chamber until the pressure in the rear chamber decreases to the second reference pressure.
In such prior art pneumatic servo booster, there are the shortcomings that in actuating the servo booster air under the first reference pressure rapidly flows through the second passage into the rear chamber with noise being generated and that in releasing the servo booster the air confined in the rear chamber rapidly flows through the second and first passages into the front chamber with noise being generated. Particularly, when the booster is mounted in a driver's compartment of a vehicle, it has been strongly required to reduce such noise.